A conventional door bell is installed on the wall at the front gate so that the visitors can press the button of the door bell which generates sound to acknowledge the owner. One of the conventional door bells is shown in FIGS. 4 and 5, and generally includes a case with an open space in which an electro-magnetic valve 1 is located. Two hit pins 2 are controlled by the electro-magnetic valve 1 and two springs 3 are mounted to the hit pins 2. When the button (not shown) of the door bell is pressed, the electro-magnetic valve 1 is activated and the two hit pins 2 hit the sound plate 4 which generates sound. The sound generated by the sound plate 4 is amplified via two sound boxes 5 which are located on two sides of the electro-magnetic valve 1. The sound can be transferred to a long distance to ensure that the owner of the house can hear the sound from the door bell.
However, it is noted that the electro-magnetic valve 1 can only be activated by direct current (DC) of 16 Voltage and the power supply of the house is alternative current (AC) of 120 Voltage, so that an adaptor 6 is equipped to transfer the current to be usable for the electro-magnetic valve 6. The adaptor 6 is huge and projects out from the case as shown in FIG. 4, and the adaptor 6 makes the door bell to have an awkward profile. Because of the bulky adaptor 6, the difficulty for installation of the door bell is increased. Furthermore, the adaptor 6 makes the door bells occupy much larger space during transportation.
The present invention intends to provide a door bell which includes a circuit board to replace the electro-magnetic valve and the door bell of the present has a compact size without any part projecting from the door bell.